Traffic Signal Justifications

There are nationally applied standards, or "warrants," for determining whether a traffic signal should be installed. If one or more of the following 9 warrants is met, a signal may be installed after an engineering study of traffic conditions, pedestrian use, and physical characteristics of the location has been done. The results of the study must show that the signal will improve the overall safety and operation of the intersection.

Eight-hour vehicular volume/minimum volume
If a certain number of cars use the main street and the side street for eight hours within a 24-hour period, then a signal is justified. The number of cars needed varies with the number of lanes. For example, if the main street and the side street each have two lanes, there have to be 600 vehicles an hour using the main and 200 an hour using the side street.

Interruption of continuous flow
There has to be so much traffic on the main street that there are no gaps in traffic to allow drivers from the side street to pull out. For the same two lane roadways above, the warrant sets traffic minimums of 900 vehicles an hour on the main street and 100 on the side street.

Four-hour-vehicular volume
This allows a signal if there is very heavy volume on both the side and main streets for four hours a day. This is a sliding scale, where the more volume there is on the main street, the less volume is required on the side street to meet this warrant.

Peak-hour volume
This one allows a signal if there are high volumes of traffic for one hour a day. This is a formula that can justify a signal if traffic on the side street is held up a certain amount of time.

Pedestrian volume
If there is a sufficient amount of pedestrian traffic crossing the main road, a signal can be installed. There must be 100 pedestrians an hour in any four hour period in a day or 190 pedestrians in a one hour period.

School crossingThis is pretty flexible. If gaps in traffic are not adequate for groups of students to cross, a signal can be put installed.

Coordinated signal system
A signal can be used to keep traffic grouped together. A signal can be added to a network of signals, if it is expected to make the system work better. It is not used locally to justify signals

Accident experience
If trial alternatives and enforcement fails to reduce crashes and there are five or more accidents over 12 months that can be remedied by a signal, and the vehicle counts are 80 percent of those used in 1, 2 and 3.

Roadway Network
Installing a traffic control signal at some intersections might be justified to encourage concentration and organization of traffic flow on a roadway network.

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